In various semiconductor fabricating processes the semiconductor substrate or wafer is heated. Usually it is important to heat the wafer uniformly across the entire mass and surface of the wafer. In certain processes, such as the chemical vapor deposition of epitaxial silicon films, nonuniform heating can result in nonuniform deposition
In chemical vapor deposition systems, it has been highly desirable to carry out the deposition reaction in a cold-wall type reaction chamber to permit transmission of radiant heating energy through the reaction chamber walls and also to avoid film deposition on the chamber walls. Cold-wall systems were additionally desirable because they permitted the deposition of high purity films, such as epitaxial silicon and silicon dioxide films. In hot wall systems, impurities could be evolved from, or permeate through the heated reaction chamber walls. Because such impurities would interfere with and adversely affect the purity of the coating, and cause particulate contamination, cold-wall reaction chambers were employed to preclude such impurity diffusion or permeation and particulate precipitation.
Chemical deposition processes have been developed formerly which permit heating of a substrate positioned within a reaction chamber without simultaneously heating the reaction chamber walls. Initially, processes such as epitaxial silicon employed the use of radio frequency (RF) induction heating of a conducting susceptor positioned within the reaction chamber, the walls of which were formed of nonconducting or insulating material, such as quartz.
However, such RF heating techniques, while generally producing the stated objective in a cold-wall reaction chamber, had several inherent and important disadvantages which make RF heating undesirable under many circumstances. For example, RF generators are very expensive to manufacture and to maintain, as well as being very large in size thereby consuming large areas of expensive floor space and must be located close to the epitaxial reactor. Moreover, the high voltages required for the RF coils produced substantial personnel hazards, was energy inefficient and frequently interfered with adjacent electronic equipment.
Subsequently, infrared (IR) radiant energy of shortwave length was developed to overcome the inherent disadvantages of RF heating. IR heating utilized high intensity, high temperature lamps generating IR of approximately one micron which could pass through quartz with minimal absorbance of heat and thus maintain the cold wall criteria of a chemical vapor deposition (CVD) reactor. The lamps were powered through solid state power supplies and controllers thereby eliminating the use of RF. Like RF heating, a susceptor was required inside the reaction chamber to support the substrates to be coated and to absorb the transmitted IR energy similar to the indirect RF coupling with the susceptor. However, experience has demonstrated that the IR heating method, though supplanting the RF method, had a number of its own inherent disadvantages. Those disadvantages found in common with the RF method included expensive manufacture and maintenance, energy inefficiency and requiring a susceptor for substrate support and energy absorption. Additional disadvantages unique to the IR method include temperature non-uniformity, the necessity in rotating the susceptor and substrates, streaking of substrate surfaces, high lamp failure rate, low system reliability and the inability to automate the process step. The process automation nonuniform heating of larger substrates utilizing IR energy presents a greater problem in newer wafer fabrication processes utilizing large wafers such as 6 and 8 inches in diameter as contrasted with the older conventional 3, 4 and 5 inch diameter wafers.